Current refrigeration systems used in transport applications, specifically truck and trailer refrigeration units, have designs which allow oil viscosity to get very high as the refrigeration compartment progressively cools. Refrigeration systems continue to run, even with the compressor capacity reduced to the fullest extent, reducing the refrigeration compartment up to 60.degree. F. below the frozen set point. As a result, oil viscosity is caused to climb to 6000 cSt or higher. This extremely high oil viscosity has a negative impact on compressor performance.
Specifically, the compressor suction valve suffers from reduced reliability. The very cold, highly viscous oil of current systems causes adhesion between the suction valve and the valve plate, thus delaying the valve opening until the suction pressure is high enough to overcome the adhesion force. Upon valve opening, the suction valve experiences a high initial velocity and momentum, causing the valve to contact the valve stop with higher force than normal or than preferred. Upon contact with the stop, the valve bends into the cylinder, thus increasing stress which can ultimately lead to valve failure.
There is a need therefore, for a system for controlling/limiting compressor oil viscosity at a desired level in refrigeration systems, specifically those used in transport refrigeration.